An open secret is a concept or idea (often in, but not necessarily limited to, government and military circles) that is "officially" secret or restricted in knowledge, but is actually widely known; or refers to something which is widely known to be true, but which none of the people most intimately concerned is willing to categorically acknowledge in public.

Examples of military open secrets:

• Area 51
• Delta Force
• Nuclear weapons and Israel
• SOSUS

An example of open secret in politics: it may be widely known that an individual government minister holds a particular opinion, but is at present unable to express that opinion publicly because it is contrary to the formally expressed view of the government of which he or she is a member. This article is about the U.S. Air Force installation in Nevada. ... The 1st Special Forces Operational Detachment-Delta (1st SFOD-D) — commonly known as Delta in the U.S. Army, Delta Force by civilians, and Combat Applications Group by the Department of Defense — is a Special Operations Force (SOF) and an integral element of the Joint Special Operations Command (JSOC). ... Israel was the sixth country in the world to develop nuclear weapons[2] and is one of four nuclear-armed countries not recognized as a Nuclear Weapons State by the Nuclear Non-Proliferation Treaty (NPT), the other three being India, Pakistan and North Korea,[3] and the International Atomic Energy... SOSUS, an acronym for SOund SUrveillance System, is a chain of underwater listening posts located across the northern Atlantic Ocean near Greenland, Iceland and the United Kingdom—the so-called GIUK gap. ...

The term has also been applied to public knowledge of a person's sexuality. For example, the actor Tom Cruise is a homosexual, but has never identified himself as such. Nevertheless, it is widely known and was thus an open secret.^[1] Tom Cruise (born Thomas Cruise Mapother IV on July 3, 1962) is an Academy Award-nominated, Golden Globe Award-winning American actor and film producer. ...

References

1. ^ Philip Brett, 'Musicality, Essentialism, and the Closet', Queering the Pitch: The New Gay and Lesbian Musicology (London: Routledge, 1994), p18.